1. Field of the Invention
The present invention relates to a uni-morph, bi-morph or other types of piezoelectric and/or electrostrictive film elements which generate or detect displacement or force in the form of bending, deflection or flexure, and which can be used for actuators, filters, display devices, transformers, microphones, sounding bodies (such as loud speakers), various vibrators, resonators, or oscillators, discriminators, gyros, sensors and other components and devices. The present invention is also concerned with a method of producing such piezoelectric and/or electrostrictive film elements. The term "element" used herein means an element which is capable of transducing or converting an electric energy into a mechanical energy, i.e., mechanical displacement, strain or vibrations, or converting a mechanical energy into an electric energy.
2. Discussion of the Related Art
In recent years, in the fields of optics and precision positioning or machining operations, there has been an increasing demand for an element whose displacement is controlled for adjusting or controlling an optical path length or a position of a member or component of a device, on the order of fractions of a micron (.mu.m), and a detecting element adapted to detect infinitesimal displacement of a subject as an electric change. To meet the demand, there have been developed piezoelectric and/or electrostrictive film elements (hereinafter referred to as "P/E film elements") used for actuators or sensors, which elements comprise a piezoelectric material such as a ferroelectric material, and utilize the reverse or converse piezoelectric effect to produce a mechanical displacement upon application of an electric field to the piezoelectric material, or utilize the piezoelectric effect to produce an electric field upon application of a pressure or mechanical stress. Among these elements, a conventional uni-morph type P/E film element has been favorably used for a loudspeaker, for example.
There have been proposed ceramic P/E film elements used for various purposes, as disclosed in JP-A-3-128681 (i.e., in the co-pending U.S. patent applications Ser. Nos. 07/550,977, 07/860,128, 08/102,960, 08/384,469, 08/392,083 and 08/452,092) and in JP-A-5-49270 (i.e., in U.S. Pat. No. 5,210,455 and U.S. patent application Ser. No. 08/013,046), which were filed by the assignee of the present invention. One example of the disclosed elements has a ceramic substrate which has at least one window, and is formed integrally with a thin diaphragm which closes the window or windows so as to provide at least one thin-walled diaphragm portion or vibratile portion. On an outer surface of each diaphragm portion of the ceramic substrate, there is formed a piezoelectric/electrostrictive unit (hereinafter referred to as "P/E unit") which is an integral laminar structure consisting of a lower electrode, a piezoelectric/electrostrictive layer (hereinafter referred to as "P/E layer") and an upper electrode. The P/E unit is formed by a suitable film-forming method or process on the corresponding diaphragm portion of the ceramic substrate. The thus formed P/E film element is relatively small-sized, inexpensive, and can be used as an electromechanical transducer having high reliability. Further, this element has a high operating response, and provides a relatively large amount of displacement by application of a low voltage, with a relatively large magnitude of force generated. Thus, the above-described element is favorably used as a member for an actuator, filter, display device, sensor or other components or devices.
To produce the P/E film element as described above, the lower electrode, P/E layer and upper electrode of each P/E unit are laminated in this order by a suitable film-forming method, on the diaphragm portion of the ceramic substrate which has been sintered. The thus formed P/E unit is subjected to heat treatment (firing or sintering) as needed, so that the P/E unit is formed integrally on the diaphragm portion. A further study of the inventors of the present invention revealed that the piezoelectric/electrostrictive characteristics of the film element is deteriorated due to the heat treatment (firing or sintering) effected during the formation of the P/E unit, more specifically, the P/E layer.
That is, the P/E layer suffers from stresses due to firing shrinkage of the P/E layer or P/E unit which is in contact with the diaphragm portion of the ceramic substrate, during the heat treatment (firing or sintering) of the P/E layer. As a result, the P/E layer may not be sufficiently sintered due to the stresses, and still suffers from stresses remaining therein after firing. In this case, the P/E film element cannot exhibit its inherent piezoelectric/electrostrictive characteristics.
In order to increase the sinterability and density of the P/E layer so as to improve the piezoelectric/electrostrictive characteristics of the film element, the firing temperature of the P/E layer may be increased, or the thickness of the diaphragm portion on which the P/E layer is formed may be reduced. However, these solutions are not yet effective to improve the sinterability of the P/E layer and to eliminate the problem of stresses remaining in the P/E layer after firing thereof. That is, the stresses remaining in the P/E layer may deteriorate the piezoelectric/electrostrictive characteristics of the film element. In particular, such residual stresses may reduce the amount of displacement of the diaphragm portion upon actuation of the P/E unit. The reduction in the thickness of the diaphragm portion makes it difficult to produce the ceramic substrate, and lowers the resonance frequency of the P/E film element.
The P/E film element suffering from such residual stresses is not capable of providing sufficient bonding strength between the diaphragm portion of the ceramic substrate and the P/E unit (the lower electrode). As a result, the P/E unit may be separated or peeled off from the diaphragm portion of the ceramic substrate during production and use of the P/E film element. Accordingly, the operating reliability of the element is undesirably deteriorated.